An electronic ballast is a device which applies a high voltage across a fluorescent lamp to initiate an arc and start the lamp, and to limit current through the fluorescent lamp after the arc is initiated.
Dimming electronic ballasts are known in the art. Such dimming ballasts typically respond to a control element which communicates over a low voltage hardwired interface to the dimming ballast to reduce the intensity of light emitted from a fluorescent tube or tubes connected to the dimming ballast. The low voltage interface requires wiring through the walls of the building. One control element can be used to control several dimming ballasts and associated fluorescent tubes; however, if it is desired to dim one of the fluorescent tubes, all fluorescent tubes attached to the control element must be dimmed. If it is desired to add new electronic ballasts and fluorescent tubes in a building, and have them controlled by the same control element, low voltage control wires from the control element to the new ballast must be installed. If it is desired to separately control the light intensity of two different fluorescent tubes, or two different sets of tubes, separate control elements are required, along with separate sets of wires from the control elements to the dimming ballasts. A dimming ballast and associated control element are fairly expensive; e.g., approximately $250 for both. If it is desired to separately control portions of a building, such as when certain sections of the building are empty of occupants, separate control elements are required, resulting in great expense.
U.S. Pat. No. 5,051,720, issued to Kittirutsunetorn on Sep. 24, 1991 and incorporated herein by reference, discloses a system for accessing a remote site electrical device using a modem and then accessing a power line. The power line interface uses slave devices to communicate with a master control, which is a remote computer. This is a master-slave control system, which is expensive and cumbersome to install and maintain.
U.S. Pat. Nos. 5,055,746, 5,068,576, and 5,107,184, issued to Hu et al. and incorporated herein by reference, disclose use of a power line interruption code to control dimming of an electronic dimming ballast. The disclosed method does not provide a complete solution and the method is complex. U.S. Pat. No. 5,264,823, issued to Stevens on Nov. 23, 1993 and incorporated herein by reference also discloses a power line communication system, but does not provide a system as contemplated by applicants.